It has been proposed to make shoes by a so-called string lasting process in which an upper is closed, a draw string is attached to the periphery of the sole attaching margin as by overlocked thread, the upper is transferred to a foot form of a sole molding machine and the draw string is tensioned to bring the roughened area of the upper to a bottom surface of the mold. Subsequently the foot form is presented to a mold cavity of the sole molding machine, and hot polyvinyl chloride compound is injected into the closed cavity to form a sole attached to the shoe upper.
With many shoe upper materials the union between the shoe upper and the molded-on sole is not sufficiently strong. To overcome this weakness, U.S. Pat. to Chandler et al No. 3,522,343 of July 28, 1970 has proposed that a thin sheet of heat activatable adhesive be disposed on the attaching surface of the bottom of the shoe upper before molding on the sole. When hot plastic sole forming material is molded against the bottom of the shoe, the adhesive is activated and establishes a firm bond between the molded on sole and the attaching surface of the shoe.
Desirably, an adhesive for commercial use in film form in a string lasting process for adhering the sole to the shoe upper, should satisfy at least some of a number of criteria to some extent, including for example capacity to be produced as a handleable film (preferably a self-supporting film requiring no interleaving paper), ease of activation, to an extent sufficient to adequately wet the upper, by molten PVC which is frequently injected at a temperature of the order of 170.degree.C. to 180.degree.C., the ability to provide a bond which when warm is of adequate strength, and rapid build up of adhesive strength to provide a firm bond showing toughness, flexibility and no substantial creep. Desirably also, the adhesive will form acceptable bonds to leather uppers made from materials of various qualities.
Whilst various polyester based, and polyurethane based adhesives have been proposed for use in the shoe making and other trades, not all of them can be provided in the form of films with appropriate handling, heat activation and other properties.